Xanthines

Mix a few mg of the substance under examination or the prescribed quantity with 0.1 ml of hydrogen peroxide solution (100 vol) and 0.3 ml of 2 M hydrochloric acid, heat to dryness on a water-bath until a yellowish red residue is produced and add 0.1 ml of 2 M ammonia; the color of the residue changes to reddish violet.

Zinc Salts

A. Dissolve 0.1 g of the substance under examination in 5 ml of water or use 5 ml of the prescribed solution. Add 0.2 ml of sodium hydroxide solution; a white precipitate is produced. Add a further 2 ml of sodium hydroxide solution; the precipitate dissolves. Add 10 ml of ammonium chloride solution; the solution remains clear. Add 0.1 ml of sodium sulfide solution; a flocculent, a white precipitate is produced.

B. Dissolve 0.1 g of the substance under examination in 5 ml of water or use 5 ml of the prescribed solution. Acidify with dilute sulphuric acid and add one drop of a 0.1 percent w/v solution of cupric sulfate and 2 ml of ammonium mercurithiocyanate solution; a violet precipitate is formed.

C. Dissolve 0.1 g of the substance under examination in 5 ml of water or use 5 ml of the prescribed solution. Add 2 ml of potassium ferrocyanide solution; a white precipitate, insoluble in dilute hydrochloric acid, is produced.